As a next-generation optical disc of a DVD, which is becoming popular, a Blu-ray Disc is under development. In the Blu-ray Disc, a blue laser beam is used to record and reproduce record data, wherein the blue laser beam has a shorter wavelength than that of a red laser beam, which is used to record and reproduce the record data on a DVD. Moreover, by shortening the pitch of tracks formed on a recording surface, the Blu-ray Disc attains a recording capacity of about 23 GB in a signal recording layer (refer to patent documents 1 to 3).
On the other hand, a sub-picture, such as a small window, can be displayed with it superimposed on a main picture (in other words, superimposed on one portion of the main picture), such as a movie, recorded on the optical disc described above. As the sub-picture, there are a sub-picture of a Synchronous type (hereinafter referred to as a “Sync type”) and a sub-picture of an Asynchronous type (hereinafter referred to as an “Async type”), in accordance with its display method. In the sub-picture of the Sync type, a display start time point and a display end time point are set in advance with respect to a reproduction time point of the main picture. On the other hand, in the sub-picture of the Async type, the display can be arbitrarily started, in accordance with a user's instruction or the like.
In this case, in order to identify the sub-picture currently selected as the display target from a plurality of types of sub-pictures recorded on the optical disc, a stream number of the selected sub-picture is held in a status register or the like. The stream number held in the status register continues to be held in the status register as long as a new sub-picture is not selected.    Patent document 1: Japanese Patent Application Laid Open NO. 2004-304767    Patent document 2: Japanese Patent Application Laid Open NO. 2004-328450    Patent document 3: Japanese Patent Application Laid Open NO. 2005-269595